1. Field of the Invention
This invention relates to coating compositions for plastics and heat-sealable plastic films coated therewith. More particularly, it relates to a heat-sealable plastic film coated with a coating composition comprising an aqueous dispersion containing a specific copolymer and which possesses excellent heat-sealing characteristics.
2. The Prior Art
Plastic films such as polyester films, polyolefin films and the like have superior transparency and physical or chemical strength, but do not have superior heat-sealing or gas barrier properties. To improve such properties of plastic films it has heretofore been proposed to coat same with a solution of a thermoplastic copolymer such as vinylidene-chloride resin in various kinds of organic solvents. While such a copolymer can readily give improved adhesion to the base film, solvents to be used for dissolving the copolymer are usually very volatile and flammable thereby inducing a danger of ignition or explosion. Moreover, some of the organic solvents which may be so utilized generate toxic vapors, thereby causing other problems. In addition, a trace amount of the organic solvent used tends to remain in the coating layer since its removal in coating procedures is difficult. Such remaining organic solvent should be avoided, particularly where the coated plastic film is used as packaging for foods or the like. Accordingly, in order to eliminate the remaining solvent, it is necessary to use large scale drying apparatus which is, however, uneconomical.
On the other hand, if an aqueous dispersion of the copolymer to be coated is prepared, the above deficiencies in the properties of the resulting plastic film resulting from the presence of residual solvent and in its preparation may be solved. However, in the case of the use of a plastic film whose surface is only subjected to a corona discharge or oxidation agent treatment, it is generally difficult to achieve sufficient adhesion between the base film and the copolymer used by application of an aqueous copolymer dispersion directly on the treated film surface. Therefore, the base film is usually pre-treated with an anchor coating or sub-coat. This is also uneconomical because of the necessity for plural coating operations.
Accordingiy, it has also been proposed to obtain sufficient adhesion by the direct-coating of plastic films with an aqueous polymer dispersion. Thus, for example, the use of a mixture of an aqueous alkyl acrylate dispersion and methylol melamine or methyl-etherified methylol melamine (Japanese Patent Publication No. 42,240/1971 has been suggested for this purpose). The resulting coating layer would improve adhesion to the base plastic film but would deleteriously affect the thermoplasticity or heat-sealing characteristics which are important basic properties of the plastic film.
A further attempt to improve such coated films involved copolymerizing an acrylate copolymer with an .alpha.,.beta.-unsaturated carboxylic acid such as acrylic acid, and neutralizing the resulting free carboxylic acid copolymer with a base such as ammonia to yield a water-soluble copolymer, to improve adhesion of the coating (U.S. Pat. No. 3,753,769). The resulting layer, however, is deficient in that it is generally weak in mechanical strength or tends to be subject to blocking. Therefore, such a method would not provide a coated film possessing the fundamental and essential characteristics of durability and blocking resistance. In addition, the coating composition employed in said method is deficient in that the copolymer may not be incorporated therein in a high concentration, since it is soluble in water.